1. Field of the Invention
This invention relates to an engine output control system for a vehicle.
2. Description of the Prior Art
In a so-called traction control system, the engine output is reduced independent from the amount of depression of the accelerator pedal of a vehicle when the slip of the driving wheels of the vehicle reaches a predetermined value so that the slip of the driving wheels converges on a target value. However when the engine output is reduced at a low engine speed, the engine can stall.
In order to overcome such a problem, in a traction control system disclosed in Japanese Unexamined Patent Publication No. 60(1985)-121129, reduction of the engine output is inhibited when the engine speed is lower than a preset engine speed (e.g., 600 to 2000 rpm). That is, in the traction control system, slip of the driving wheels is detected on the basis of the difference between the wheel speed of the driving wheels and that of the driven wheels, and when the slip of the driving wheels thus detected reaches a predetermined value, fuel supply to one or more of the cylinders of the engine is cut to reduce the engine output so that the slip of the driving wheels converges on a target value. When the engine speed is lower than a preset engine speed, fuel cut is inhibited.
Though the traction control system is advantageous in that the engine is prevented from stalling since fuel cut is inhibited when the engine speed is low, it gives rise to the following problems due to the fact that the preset engine speed is fixed.
That is, when the engine output reduction requirement is large, for instance, to such an extent that fuel supply to all the cylinders is to be cut, the engine can stall since the preset engine speed is relatively low for the engine output reduction requirement and the engine output is abruptly lowered. On the other hand when the engine output reduction requirement is small, for instance, to such an extent that fuel supply to only one cylinder is to be cut every other intake stroke, the engine output cannot be sufficiently reduced though the engine is not apt to stall in such conditions since the preset engine speed is relatively high for the engine output reduction requirement.